If you're perfectly average, it's probably a decent guideline, but if you're very athletic with more muscle than the average, then the BMI guidelines are too low. If you're more sedentary and undermuscled than average, the BMI guidelines would be a bit high.

BMI guidelines are probably a bit generous for me right now because I'm not very active, but I'm working on that. One day I would love to be more muscular than average, so that my ideal BMI might eventually (in theory) fall in the mildly overweight category.

Also, BMI isn't designed to calculate ideals, just averages, so I think at best it's a ballpark starting point, and nothing more. Finding your own unique best weight and best activity/athletic level requires further tweaking/trial and error.